2,3-Pyridinedicarboxylic acids are useful in the preparation of herbicidal 2-(2-imidazolin-2-yl)nicotinic and quinoline-3-carboxylic acids, esters and salts. In the manufacture of these herbicidal compounds, aqueous media containing undesirable 3-(2-imidazolin-2-yl)picolinic acid by-products are obtained.
Heretofore, these 3-(2-imidazolin-2-yl)picolinic acid by-products served no utitity and were disposed of by incineration. Now it has been found that the undesirable 3-(2-imidazolin-2-yl)picolinic acid by-products can be recycled by conversion to 2,3-pyridinedicarboxylic acids, useful starting materials for the manufacture of herbicidal 2-(2-imidazolin-2-yl)nicotinic and quinoline-3-carboxylic acids, esters and salts, thus eliminating the need for disposal and resulting in significant environmental and cost benefits.
U.S. Pat. No. 4,658,030 describes the base-catalyzed cyclization of carbamoyl nicotinic acids and carbamoyl 3-quinolinecarboxylic acids to herbicidal 2-(2-imidazolin-2-yl)compounds. U.S. Pat. No. 4,658,030 also describes the ring opening conversion of 2-(2-imidazolin-2-yl)compounds to carbamoyl nicotinic acids and carbamoyl 3-quinolinecarboxylic acids in the presence of base. However, the patentees do not describe a method for the conversion of 3-(2-imidazolin-2-yl)picolinic acids to 2,3-pyridinedicarboxylic acids.
It is an object of the present invention to provide an efficient and effective process for the preparation of 2,3-pyridinedicarboxylic acids from undesirable 3-(2-imidazolin-2-yl)picolinic acids.